Electrical alternans relate to the differences in electrical potential at corresponding points between alternate heartbeats. T-wave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or T-wave of an electrocardiogram (ECG) which repeats itself every two beats and has been linked to underlying cardiac instability. Typically, by enumerating all consecutive heart beats of a patient, beats with an odd number are referred to as “odd beats” and beats with an even number are referred to as “even beats.” A patient's odd and even heartbeats may exhibit different electrical properties of diagnostic significance which can be detected by an ECG.
The presence of these electrical alternans is significant because patients at increased risk for ventricular arrhythmia's commonly exhibit alternans in the ST-segment and the T-wave of their ECG. Clinicians may therefore use these electrical alternans as a noninvasive marker of vulnerability to ventricular tachyarrhythmias. The term T-wave alternans (TWA) is used broadly to denote electrical alternans such as these. It should be understood that the term encompasses both the alternans of the T-wave segment and the ST-segment of an ECG.
T-wave alternans (TWA) has been demonstrated in many studies as a strong predictor of mortality, independent of left ventricular ejection fraction (LVEF). More specifically, it has become well known that T-wave alternans has predictive value for arrhythmic events such as tachyarrhythmias. Additionally, T-wave alternans has been determined to be an indicator of various forms of disordered ventricular repolarization, including disorders found in patients with cardiomyopathy, mild to moderate heart failure, and congestive heart failure.
T-wave alternans (TWA) may be caused by changes in ion exchange during repolarization. If there is a change in the repolarization mechanism on one beat, the heart attempts to readjust on the following beat. This is manifested as an alternating change in the action potential. In the surface ECG this is seen primarily as an amplitude change. For an implanted medical device such as a cardiac pacemaker, the intracardiac electrogram (IEGM) also shows a change in timing. Thus, the term T-wave as used herein may refer to a portion of the ventricular QRS-T-wave complex that includes the T-wave and the QRS-T segment. The alternating feature of TWA can be detected by examination, for example, of the QT interval, T-wave width, T-wave amplitude and morphology, etc. Whatever the designated portion of the intracardiac electrogram, T-wave alternans refers to an alternating pattern of the wave that can be designated “A-B-A-B-A . . . ” where A represents every other cycle and B represents every other alternate cycle. As discussed in the literature, when such an alternating pattern appears, the different rates or forms of repolarization of the ventricular cells are statistically associated with a variety abnormal cardiac conditions. Further, the alternating repolarization pattern can lead to increased instability and consequent cardiac arrhythmias. Thus, the presence of T-wave alternans is recognized as an indicator of risk for ventricular arrhythmia and even sudden cardiac death (SCD).
In the past, detection of T-wave alternan patterns has been performed using surface ECGs. Implementation of such detection has included the measurement, on a beat-to-beat basis, of the micro-volt level changes in the T-wave amplitude from the surface ECG. Then, the long record of time series of T-wave amplitude change is transformed into the frequency domain by Fourier series transformation (FFT). A prominent peak in the FFT at 0.5 Hz would verify the existence of a T-wave alternan pattern.
Unfortunately, the above detection method requires the use of medical equipment that must be operated by medical personnel in a medical facility such as a physician's office. The detection requires long term recording of surface ECGs and off-line analysis with robust computation equipment. As a result, T-wave alternan pattern monitoring has been inconvenient and cumbersome. As a result, it is difficult to provide continuous and regular T-wave alternan pattern monitoring.
Many patients who would benefit from T-wave alternan pattern monitoring have an implanted cardiac device such as an implantable defibrillator or a combined defibrillator pacemaker. It would thus be highly desirable if such an implanted device could monitor for T-wave alternan patterns. However, the prior art detection method does not lend itself for such application due to, for example, the required long term monitoring, surface ECG, and robust computational requirements for Fourier series transformation.
In order for an implanted cardiac device to provide T-wave alternan pattern monitoring, there is a need for a new and different approach. The present invention addresses that need.